This invention relates to a print head for electrostatic printing of documents in accordance with the so called aperture matrix technique, which usually is called DCF-technique being an abbreviation of Digital Control Filter technique, which print head includes means for charging of tuner particles with a certain polarity, a tuner depositing matrix which is provided with a plurality of apertures, trough which the toner particles can pass, means for controlling the deposit of tuner particles to certain specific depositing positions, and means for feeding a document, which in to be printed under the aperture matrix.
During approximately the last 10 to 15 years work has proceeded to simplify the so-called laser printer technique. Characteristic of his technique is that a digitalized picture is applied on a rotating, light sensitive drum by means of digitalized light. Then different electrical levels on the drum surface will represent the picture, subsequently the picture is developed by electrically charged powder, and finally the picture is moved to the document by pressure transfer. Pressure and heat between a pair of rollers fix the digitalized picture now existing on the document.
The characterising feature of this printing technique is that a pulverised dry ink is used, which generally is called a toner and is fixed to the documents by a fuse-pressure process, whereby the document is provided with an excellent durability for a long time and resistance to external influence by mechanical wear and moisture etc.
The present simplifying proceedings is to eliminate the previous intermediate step of storing the picture on a drum, where it is developed and later moved to the document This involves unnecessary costs due to additional components and an increased risk for distorting the digital information on its way from the computer to the state, in which is fixed on a document.
The most recently developed laser printers provide a picture quality of 600 DPI (dots per inch) or better. To cover an A4 page with all possible dot positions 34 million positions are required at a resolution of 600xc3x97600 DPI. The problem is to find a technique by which it is possible to apply a powder dot in the shape of a small dot on the correct location on the document, when there are 34 million possibilities.
The most successful technique during the last few years is to use a so-called xe2x80x9cDigital Controlled Filterxe2x80x9d (DCF), henceforth called a matrix or an aperture matrix, comprising a row of small apertures in a thin foil. The apertures are opened or closed by application of an electrical voltage, which generates a field that attracts the electrically charged powder to pass or not to pass through the apertures. On one side of the foil the document is situated, upon which the particles land, and on the other side of the foil the charged powder is situated on a conveyor. In most applications this conveyor consists of a roller with a thin layer of electrically charged powder.
This technique was described for the first time in the U.S. Pat. No. 3,689,935 to Pressman and Casanova in Sep. 5, 1972. Several later patents disclose various improvements of the Pressman/Casanova patent, e.g. U.S. Pat. No. 4,491,855 (Fuji/Andoxe2x80x94Canon from 1985), U.S. Pat. No. 4,912,489 (Schmidlinxe2x80x94Xerox from 1990) and Swedish patent No. 8704883-1 (Ove Larssonxe2x80x94Array Printers from 1989). The various improvement methods disclosed in these patents relate to how the electrical field around each aperture is appplied to obtain a more efficient transfer of the powder through the apertures down to the document.
Problem
An aperture matrix may comprise a row of apertures covering the width of an A4 sheet, i.e. 210 mm or a row of 4960 apertures to create a 600 DPI print-out. The advantage with this type of aperture matrix is that printing can be done with continuous paper feed and with continuous fixing, and in other words a continuous printing process is obtained.
There are many problems connected with an aperture matrix being as wide as said 210 mm.
1. If every aperture is connected with a drive circuit for the application of a voltage on a round electrode, which is arranged around the aperture, this involves several thousands ring electrodes, connection lines and drive circuits, and with increasing number of apertures in the aperture matrix the probability increases for breakdown in any of these apertures, rings, connection lines or drive circuits.
2. If the control field for opening and closing the apertures in the aperture matrix is applied between ring electrode and powder feed surface there is an additional problem, for the distance between the aperture matrix and the powder feed must have the magnitude of 0.1 to 0.15 mm, since available drive circuits can stand a maximum of 300 to 350 volt. Therefore this distance is not allowed to vary more than 0.02-0.04 mm since the variation of the distance has an influence on the electrical filed, which in turn has an influence on the blackness of the printout. Since the aperture matrix is a thin foil it is very difficult to maintain a distance of 0.1 to 0.15 mm within the said tolerance between the aperture matrix an the powder feed surface over the entire width of an A4 sheet.
3. According to paragraph 2 drive circuits are required, which can stand at least 300 volts and preferably more, and such drive circuits in the form of integrated circuits are still too expensive, which creates a cost problem. The solution of the cost problem is to use lower voltage or fewer circuits.
4. A further problem is that there exists a certain percentage of powder, which not will be charged or charged to the opposite potential. This powder can not be controlled by electric control fields but will fly around without control or deposit on undesirable surfaces.
Common types of printing powder will be electrically charged with negative potential, when the powder particles rub against some other suitable material. This rubbing is usually created in two ways, either with a rotating magnetic field in case the powder is provided with magnetite and a rotating magnetic brush is created, which rub the particles against a roller. In case the powder lacks magnetite a rotating brush or a foam rubber roller, which rotates against a steel roller, rubs the powder particles against this steel roller. In booth cases the correct amount of powder is dosed on the rotating steel roller by means of a flexible rubber blade. This dosing, called doctoring, has a grinding influence on the powder, and the disintegration of powder particles causes creation of particles, which are booth incorrectly charged and uncharged.
The four main problems mentioned above are the reason for the fact that the technique mentioned above not yet has been any great commercial success, though it offers direct transfer of digital information from a computer to a document.
The Invention
Our proposal is to use an aperture matrix comprising en isolating foil with a completely covering, electrically conducting surface on one side of the foil (called the base side). On the other side of the foil there are a greater number of electrically conducting rings (called ring side), each of which is formed around an aperture through the aperture matrix. Each ring is connected to a voltage source through a separate line to a control transistor. The base side of the foil has a common potential, since it has a completely covering, electrically conducting surface.
According to the invention it is proposed that the electrically charged powder now is applied above the base side on the aperture matrix as a cloud of charged particles, which constantly are moving. This cloud is created in such a way, that toner which is situated in a toner container, is fed forwards by a feeding brush, which rotates clockwise (in the case shown in FIG. 2) with a low number of revolutions to correspond with the consumption of tuner in the printing process.
Tangential to the feeding brush is a flipping brush, which rotates anticlockwise with a higher number of revolutions than the feeding brush. In the tangential point between these brushes stirring of the particles occur due to the difference in number of rotations between the brushes.
The mission of the feeding brush is to control, dependent on its number of revolutions, that the amount of tuner in the flipping brush is on the right level in order to guarantee that the toner concentration in the toner cloud not varies due to variations of the amount of tuner in the flipping brush.
To provide a negative charge of tuners, the fibres of either the flipping brush or in booth the feeding brush and the flipping brush consist of a material, which is tribo-electrically positive with respect to the toner. When the particles collide with the brush fibres in the moment of stirring, the fibres become positively charged and the toner negatively charged. The flipping brush gets a positive charge and the toner particles will stick to the brush straws. The flipping brush will continue to rotate, with the straws filled with toner particles, on to a metal blade, which bends the straws backwards, and when the straws pass the edge of the blade, the straws will flip and become straight, whereby the toner particles are shaken off and a toner cloud is formed. In this toner cloud the particles move downwards towards the aperture matrix.
It is important that the flipping brush is situated on the same centre line, which extends through the centre of the aperture zone in the aperture matrix, so that the toner cloud, which is formed, will be as homogeneous as possible over the entire width of the aperture zone. The distance between the periphery of the flipping brush and the upper side of the base side of the aperture matrix should not exceed 0.5 mm. This distance is of importance to maintain correct toner density in the toner cloud.
The particles, which are not used in the printing process, may either mechanically bounce back to the flipping brush or by means of electronical forces move back to the flipping brush, which is positively charged, and also up towards the edge of the flipping brush housing, which also is positively charged.
Due to the fact that the flipping brush is semi-resistive and the central shaft of the flipping brush is connected to a positive voltage, there exists a field between the base side of the aperture matrix and the fibres of the flipping brush. Now this field shall have such direction, that it lifts the negatively charged toner back into the brush. This returning force must be of the same order of magnitude as the flipping force, so that approximately the same amount of tuner returns to the brush as the amount of tuner coming out from the brush reduced by the amount of tuner which is transported downwards through the apertures. Also the front edge of the brush housing surrounding the flipping brush, is preferably connected to the same positive voltage as the centre shaft of the flipping brush, so that tuner, which not manages to return to the brush, will get stuck there and successively is fed back into the brush.
As an alternative proposal the centre shaft of the flipping brush is connected to en alternating voltage being 180xc2x0 out of phase in relation to the printing process. When the apertures are open practically no toner is returned to the brush. When the apertures are closed, however, maximal amount of tuner is returned to the brush. This function is controlled by the computer card of the print unit, so that these fields are synchronised (see FIG. 3).
These toner particles will now follow the flipping brush back to the tangential point between the brushes, where the particles are mixed with new particles and recharged.
Booth brushes are surrounded by a housing, which tightly fits around the periphery of the brushes. The reason for this is that toner, being fed forwards by the brushes, shall advance and that no tuner shall spatter out from the brushes due to centrifugal force when the brushes rotate. Thereby control is secured that all charged tuner will arrive in the proper place. The housings around the brushes also provide a possibility of changing the brushes when the toner container is changed. Thereby a new set of brushes will be provided every time the toner container is changed, and the risk of worn out and damaged brushes is reduced.
On the ring side of the aperture matrix the document is situated, and under the document there is a back electrode, which is connected to a positive potential of 1000-2000 volts. Between this back electrode an the base side of the aperture matrix, which has the level of zero volt, there is formed an electric field, called the main field, through the apertures in the aperture matrix, and around the aperture edge on the base side of the aperture matrix an edge field will be formed. This edge field is directed in such a manner, that toner particles moving down through the aperture when the ring around the aperture on the aperture side is at zero volts. If the ring around the aperture is at minus 100-200 volts, the edge field gets the opposite direction, whereby a barrier field is created.
The toner particles in the toner cloud are not able to move downwards through the apertures.
The toner particles, which are located in or closely below the aperture, will now move upwards through the hole back into the toner cloud, and the aperture will be blocked for tuner in the cloud.
If the ring is at zero volts, the main field will accordingly drive tuner from the cloud down through the aperture to the document, but if the ring has a negative potential of 100-200 volts the blocking field will block transport of tuner, and no tuner will pass through the aperture.
The patent U.S. Pat. No. 5,404,159 discloses an arrangement, by means of which printing is provided by a toner cloud. This prior art arrangement, however, is different from the present invention mainly through the way a stable tuner cloud is created:
in the present arrangement there is firstly used a feeding brush, the task of which is to feed a controlled amount of tuner to a flipping brush, so that the flipping brush receives the correct toner concentration to cerate a prerequisite for the correct toner concentration in the toner cloud;
the flipping brush is contacting the feeding brush to create a charge between the brush fibres of the flipping brush and the toner, so that toner shall stick to the brush fibres by electrical force;
the flipping brush is centrally arranged with respect to the aperture zone to create a homogeneous toner cloud in relation to all aperture positions in the aperture matrix;
the flipping brush is semi-resistive and connected to a positive potential to create en electric field between the upper side of the aperture matrix and the brush, that actively feeds such tuner back which not has been used in the printing process;
the brushes are enclosed in a housing, the task of which is to secure that all of the tuner arrives at the means for generating the tuner cloud, whereby the housing contributes to the achievement of a stable toner concentration in the toner cloud.